1. Field of the Invention
The present invention relates to a 4-point contact ball bearing, and in particular to a 4-point contact ball bearing that can suppress increases in differential slip between balls and raceway surfaces when the contact angles between the balls and the raceway surfaces change due to machining errors in the raceway surfaces and the like.
2. Related Art
As shown in FIG. 3, a typical 4-point contact ball bearing 100 has an outer ring 102, an inner ring 103, and balls 105. The balls 105 are inserted into a ring-shaped raceway formed between an inner circumferential surface of the outer ring 102 and an outer circumferential surface of the inner ring 103 in a manner that 4-point contact can be established. The ring-shaped raceway is defined by a pair of ring-shaped raceway surfaces 106, 107 of the outer ring 102 and a pair of ring-shaped raceway surfaces 108, 109 of the inner ring 103. When a cross-section is taken of the 4-point contact ball bearing 100 in a plane that includes a central axis of rotation, the profile of each ring-shaped raceway surface is defined by an arc that can be circumscribed around the balls 105.
Centers of curvature 106a, 107a, 108a, and 109a of these ring-shaped raceway surfaces 106 to 109 are displaced with one another, so that when a radial load is applied, the balls 105 contact the four ring-shaped raceway surfaces 106 to 109. When an axial load is applied, or when the load conditions are such that the axial load is dominant, the balls 105 of the 4-point contact ball bearing 100 only contact one ring-shaped raceway surface on each of the inner ring 103 and the outer ring 102. This is the same way as when a single-row angular ball bearing is subject to an axial force.
There is only a very slight difference in the radii of curvature between the balls 105 and the arcs defining the four raceway surfaces 106 to 109. Therefore, machining errors in the raceway surfaces 106 to 109 cause large fluctuations in the contact points between the raceway surfaces 106 to 109 and the balls 105. As a result, the contact angles of the balls vary, which greatly affects the bearing performance.
Also, due to elastic deformation of the inner ring 103 and the outer ring 102, the contact between the balls 106 and the raceway surfaces 105 to 109 becomes not point contact but contact within oval surfaces 110 that have a major axis in the direction of the central axis of rotation and a minor axis in a rolling direction of the balls 105. At different positions within these contact surfaces, the distance from the central axis of rotation is not constant for the ball 105 and the raceway rings 102, 103, so that slippage occurs between the outer circumferential surfaces of the balls 105 and the raceway rings 102, 103.
Furthermore, when the contact points of the balls 105 and the raceway surfaces 106 to 109 vary due to machining errors in the raceway surfaces 106 to 109, such as when a gap of distance C is produced between contact center positions 112 and 113, the rotational slipping of the balls 105 becomes worse.